The present invention relates to a color image reflecting apparatus of a projecting type.
The basic structure of a conventional reflecting type liquid crystal image projecting apparatus is composed of a plurality of reflecting type optical writing liquid crystal light valves, writing means each for optically writing an image of each color component to each of the reflecting type light writing liquid crystal light valves by illuminating a writing light from one face side thereof, a polarized light illuminating optical system for illuminating polarized light illuminating luminous fluxes corresponding to the respective color components on reading faces of the respective reflecting type optical writing liquid crystal light valves and reflectively reading images of the respective color components written in the reflecting type optical writing liquid crystal light valves and a projecting optical system for synthesizing, magnifying and projecting the read images of the respective color components thereby projecting a color image.
First, an explanation will be given of structure of the reflecting type optical writing liquid crystal light valve that is used in the reflecting type liquid crystal image projecting apparatus.
FIG. 3 is a sectional view showing the structure of the reflecting type optical writing liquid crystal light valve. Transference electrode layers 302a and 302b and orientation film layers 303a and 303b are provided on the surfaces of transparent substrates 301a and 301b such as glass or plastics for sandwiching liquid crystal molecules. The transparent substrates 301a and 301b on their respective sides of the orientation film layers 303a and 303b are opposed while controlling the clearance by interposing spacers 309 thereby sandwiching a liquid crystal layer 304. Further, a photoconductive layer 305, a light shielding layer 306 and a dielectric mirror 307 are laminated between the transference electrode layer 302a on the side of writing by light and the orientation film layer 303a and reflectionless coating layers 308a and 308b are formed on outer faces of cells of the transparent substrate 301a on the side of writing and the transparent substrate 301b on the side of reading. As liquid crystals of the liquid crystal layer 304, nematic liquid crystals or ferroelectric liquid crystals etc. are used. Especially, a reflecting type liquid crystal light valve using ferroelectric liquid crystals is provided with a very fast operational speed of several hundreds Hz or more. Although it is known that the reflecting type light writing liquid crystal light valve using the ferroelectric liquid crystals is a device for thresholding and making binary an input image, it is also possible to perform a gray scale display by devising the waveform of a drive voltage.
In reading an image written in such a reflecting type optical writing liquid crystal light valve, firstly, a polarized light component of a luminous flux is limited to a linearly polarized light, for example, a s polarized light component formed by a polarizing plate etc. that is irradiated on the reflecting type optical writing liquid crystal light valve. Thereafter, only a linearly polarized light of a luminous flux reflected by the reflecting type optical writing liquid crystal light valve that is orthogonal to the polarization axis of the linearly polarized light of the incident luminous flux, for example, (a p polarized light component), is transmitted through a polarizing plate etc. by which the written image can be read as intensity information. The image read in such a way becomes a positive image.
Next, a specific explanation will be given of the structure of the reflecting type liquid crystal image projecting apparatus in reference to FIG. 4. This reflecting type liquid crystal image projecting apparatus is composed of three sheets of reflecting type optical writing liquid crystal light valves. That is, the apparatus includes a reflecting type optical writing liquid crystal light valve (hereinafter, R-SLM) 113 allocated with a red image among those having three elementary colors of red, green and blue, a reflecting optical writing liquid crystal light valve (hereinafter, G-SLM) 105 allocated with a green image and a reflecting type optical writing liquid crystal light valve (hereinafter, B-SLM) 110 allocated with a blue image.
This reflecting type liquid crystal image projecting apparatus includes TFT liquid crystal panels and writing lenses as writing means of the respective images of the respective color components and a red component image displayed by the R-TFT 115 is optically written on a writing face of the R-SLM 113 by the R-writing lens 114. Similarly, a green component image displayed by the G-TFT 107 is optically written on a writing face of the G-SLM 105 by the G-writing lens 106. Further, a blue component image displayed by the B-TFT 112 is optically written on a writing face of the B-SLM 110 by the B-writing lens 111.
Meanwhile, the apparatus includes as a polarized light illuminating optical system a light source 101, an illuminating lens system 102, a polarized beam splitter (hereinafter, PBS) 103, a red reflecting dichroic mirror (hereinafter, R-DM) 402 and a blue reflecting dichroic mirror (hereinafter, B-DM) 401. A luminous flux emitted from the light source 101 becomes an illuminating luminous flux irradiated on the reflecting type optical writing liquid crystal light valves by the illuminating lens system 102. The illuminating light flux is split into mutually orthogonal polarized illuminating fluxes by the PBS 103. When one polarized illuminating flux reflected by the PBS 103 is, for example, a s polarized light, the other polarized illuminating luminous flux transmitted through the PBS 103 becomes a p polarized light. Only a red component included in the s polarized light component is selectively reflected by the R-DM 402 which is irradiated on the R-SLM 113 and reflectively reads a red component image. The remaining color component transmitted through the R-DM 113 is separated into a green component and a blue component by the B-DM 401. A green component transmitted through the B-DM 401 is irradiated on the G-SLM 105 and reflectively reads a green component image.
Meanwhile, the blue component reflected by the B-DM 401 is irradiated on the B-SLM 110 and reflectively reads a blue component image. The three kinds of the red component image, the green component image and the blue component image which have been read in this way, are again synthesized by the B-DM 401 and the R-DM 402, the synthesized transmits through the PBS 103 and is magnified and projected on a screen 117 in front via a projecting lens 116. As a result, a color image is projected on the surface of the screen 117.
However, there are following problems in the reflecting type color image projecting apparatus.
In the conventional reflecting type color image projecting apparatus, the light source luminous flux is separated into a s polarized light and a polarized light component by using the PSB 103 and only the s polarized light (or p polarized light) component is taken out as a polarized light illuminating luminous flux. The other p polarized light (or s polarized light) component is not used at all as an illuminating luminous flux. Accordingly, in the conventional structure the utilization efficiency of the light source luminous flux cannot exceeds 50% and the brightness of the projected color image is low.
In addition thereto, in case where a luminous flux is not incident on a color separating mirror such as the B-DM 401 or the R-DM 402 in an orthogonal direction, there causes a deviation in the reflectance characteristic with respect to a wavelength depending on the polarized light component.
FIG. 5(a) and FIG. 5(b) are views showing the reflectance characteristics of the B-DM 401 and the R-DM 402 with respect to the s polarized light component and the p polarized light component. These drawings illustrate the characteristics in case where the luminous flux is incident on the B-DM 401 and the R-DM 402 by an angle of 45.degree.. It is very difficult and almost impossible to make the reflectance characteristics with respect to the s polarized light component and the p polarized light component agree with each other.
In the conventional reflection type color image projecting apparatus shown in FIG. 4 the R-DM 402 is provided with the characteristic shown in FIG. 5(b) therefore, the polarized light illuminating luminous flux having the s polarized light component that is reflected by the PBS 103 for illuminating the R-SLM 113 is provided with the wavelength characteristic of the s polarized light shown in FIG. 5(b). The luminous flux that is modulated and reflected by the R-SLM 113 therefore, is reflected by the R-DM 402 and transmits through the PBS 103 thereby enabling to read the image of the red component.
However, the red image of the red color component is composed of only the p polarized light component. That is, the luminous flux in compliance with the wavelength characteristic of the p polarized light is obtained by the R-DM 402. A difference of characteristics between the s polarized light and the p polarized light shown by a hatched portion of FIG. 5(b) is not utilized and becomes a total loss. Similarly, the same is applicable to the blue component reflected by the B-DM 401 and a difference between characteristics of the s polarized light and the p polarized light shown by a hatched portion of FIG. 5(a) becomes a total loss. With regard to the green component, only the s polarized light component which has not been reflected in FIG. 5 (a) and FIG. 5(b) is illuminated on the G-SLM 105. All of the wavelength band of the p polarized light component of the luminous flux which has been modulated by the G-SLM 105 can transmit through the PBS 103.
The reason is that all of the band of the s polarized light component transmitting through both the B-DM 401 and the R-DM 402 is included in the band of the p polarized light component. Therefore, a total white image projected by the conventional reflecting type color image projecting apparatus wherein the B-DM 401 and the R-DM 402 are used in a 45.degree. direction is provided with the wavelength characteristic as shown in FIG. 5(c). The light at the hatched portions in FIG. 5(c) becomes a loss. It is not possible in the conventional reflecting type color image projecting apparatus to use all of the polarized light illuminating luminous flux having only the s polarized light component. That is, an amount of light becomes a loss by a difference between the reflectance characteristics of the color separating mirrors with regard to the s polarized light component and the p polarized light component.
Further, in the conventional structure at least two sheets of the color separating mirrors (that is the B-DM 401 and the R-DM 402) are necessary between the PBS 103 and the G-SLM 105 or the B-SLM 110. That is, the back focus of the projecting lens must be set long since the optical lengths from the projecting lens 116 to the G-SLM 105 and the B-SLM 110 become long. Accordingly, the F number of the projecting lens becomes large. Meanwhile, a bright and highly-magnified projecting lens is necessary to magnify and project a color image having a high brightness. Accordingly, it is extremely difficult to design and manufacture a projecting lens which satisfies the mutually conflicting required characteristics.